Burying utility cables under the seabed in shallow water environments to ensure cable safety is accelerating because of green power generation initiatives and the relative ease of securing permits to lay cable in rivers or estuaries compared to onshore right-of-ways. In locations with shifting sands and bottom profiles, verification that the utility line remains covered is often required. A new remote sensing method has been developed to accomplish depth-of-cover utility surveys at greatly reduced cost.

Electromagnetic cable tracking is based on the detection of a magnetic field, which is generated when a low frequency electric current is placed on a metallic element or the shield of a cable. Traditionally, these methods require bringing underwater receivers close to the utility, and then finding special points in the electromagnetic field, known as the ‘peak’ or ‘null’ of the emitted signal. Once such special measurement points are known, the position and sometimes the depth can be inferred. Conventional methods to confirm cable position using electromagnetic equipment typically require a diver to work in close proximity to the buried cable to find the peak and null of the signal induced in the cable.

Advanced Electromagnetic Locating

Field tests of a new towed remote sensing instrument, called FieldSens, demonstate that the 3D geoposition of an underwater utility cable can be located in significantly less time and at lower cost. This technology is based on the optimisation of data from a variety of sensors against a physical model of the magnetic field expected from a utility cable [See sidebar technology description]. The effects of ground conductivity and field distortions are accommodated in the real-time data processing. This ‘model-based’ method eliminates the need to bring the measurement equipment to specific points related to the utility cable or pipe in order to determine the position of the cable. The FieldSens system need only be in the approximate vicinity to calculate position with appropriate level of confidence. The system uses two 3D magnetic field sensors, a tri-axial accelerometer and a digital compass to identify the offset, depth, current, and yaw angle to the underground utility or sonde regardless of its position in the radiated field. In combination with the Ashtech Proflex 500 GNSS receiver, automatic positioning occurs even from the side of the actual utility.

When the results are transferred to a GIS database, a history of such maintenance can be developed over time to determine expected areas for coverage renewal operations.

The new towed system eliminates the need to find special points, first by guiding the operator to the targeted utility from up to 40 metres away, and then accurately determining the 3D position of the line from up to 9 metres away, from any arbitrary towing direction. Furthermore, error estimates of the positioning are available, allowing real-time verification of the cover when the utility depth is compared to the sounder-based bottom depth.

Biography of the author

Rindert Procee is the Alliander resident expert in underwater cable location. He has perfected methods for undersea pipeline and cable location since 1990 in the Wadden Sea and interior canals and waterways throughout the Netherlands.Jim Waite co-founded Optimal Ranging, Inc. in 2009 to focus on precise location technologies in GNSS-denied environments. In addition to cable and pipeline location, his background includes signal processing and system development. He can be reached at jwaite@optimalranging.com.